This invention relates to particulate aluminum, suitable for use in the preparation of explosives and blasting agents. In this context, the particulate aluminum used is commonly referred to as "aluminum grain".
A "blasting agent" is an explosive material, generally comprising a mixture of at least two essential components: a fuel and an oxydiser. The agent can be either a dry powder or a slurry; slurries are generally aqueous systems. The choice of dry or slurry depends to some extent on the conditions in which the blasting agent is to be used. In modern blasting practice, dynamite and other systems related to nitro-glycerin have now almost completely been replaced by systems in which ammonium nitrate is the oxydiser, and a carbonaceous compound, e.g. a hydrocarbon oil, is the fuel. These systems, if blended properly, are both effective blasting agents, and are comparatively safe to handle, in either dry or water-wet slurry form. They are generally described as AN-FO systems, or blasting agents.
In general, dry AN-FO systems have two disadvantages: low density, and low water resistance. In an attempt to overcome these main disadvantages, the water slurry systems were developed. These slurries generally contain about 15% water. But, in their turn, although the slurries do to some extent overcome the density and water resistance problems, in that a water-wet system will still explode the slurries also have the disadvantage that they are much less sensitive to detonation than the dry systems. They therefore require the use of sensitizers or boosters, or both. Two common materials used for sensitization are paint-grade aluminum powder, which is particulate aluminum of a very fine particle size, or TNT prills. TNT, being a high explosive, adds its own peculiar handling problems. Paint-grade aluminum flake also is not free of handling problems, as is discussed more fully below.
It has also been found that the bulk strength, as an explosive, an AN-FO blasting agents can be considerably improved by the addition thereto of energetic metal fuels: for reasons of cost and availability, the commonest used is aluminum grain. Exactly how aluminum grain enters into the chemical reactions that go on during detonation is not known. There are two possibilities: it can either react with the ammonium nitrate, or it can react with water present in the system. In theory, either of these reactions will lead to the release of considerable amounts of energy, the aluminum--water reaction probably releasing the most. It has been reported that a 40/40/20 mix of ammonium nitrate/aluminum grain/water generates about 2.1 kcal/g. In view of this, aluminum grain is widely used in AN-FO systems, but not without some problems being encountered.
One common problem, which available theory does not adequately explain, is that blasting agents containing paint grade aluminum powder as sensitizer in a slurry system are known to go "dead" and not explode (either at all, or to give the required blasting effects). One theory advanced to explain this is the so-called "hot-spot" initiation process. The aluminum grains are assumed to have adhering thereto microscopic air bubbles: these bubbles come under great pressure from the initiator shock wave, and detonation is then caused by the resulting adiabatic heating. But this theory does not meet all of the available evidence. For although a "dead" system can be reactivated by aeration, but it can also be reactivated by blending in air filled glass or plastic microscopic capsules or ballotini. Further, the amount of oxide present on the grain also appears to affect its sensitivity.
But by far the major problem inherent in using aluminum grain is either sensitizer, for which a fine particle size seems to be needed, or as a metallic fuel, in compounding either dry or wet AN-FO mixes is a handling problem encountered in most of the commercially available materials. This problem is that both paint-grade powder and commercially available blasting grain, whether made by air blowing, disintegration by dropping molten metal into water, or hammer milling, contain a proportion of extremely fine powder which presents a considerable dust explosion hazard. This explosion hazard has had an adverse effect on the use of AN-FO-aluminum grain systems, even though such systems have clear advantages as blasting agents.
This problem has been studied before. One proposed solution is to coat the alunimum grain, with a thin layer of polytetrafluoro-ethylene, or a closely related polymer. Whilst this procedure is effective, it also results in much lowered sensitivity--thus calling for a more powerful initiator system--and also lowered output. By this it is meant that up to 30% more aluminum grain may be needed to achieve the same blasting effect, when compared to untreated aluminum grain. This invention seeks to provide an aluminum grain which retains both its sensitivity and power.